Refactor forkedstoragetrie.js, improve findPath callback in keyExists (…

…#378)

Clarifies the callback by removing the if statement while retaining the same logical result.

lib/utils/forkedstoragetrie.js

@@ -26,10 +26,7 @@ ForkedStorageTrie.prototype.keyExists = function(key, callback) {
   key = utils.toBuffer(key);
 
   this.findPath(key, function(err, node, remainder, stack) {
-    var exists = false;
-    if (node && remainder.length === 0) {
-      exists = true;
-    }
+    const exists = node && remainder.length === 0;
     callback(err, exists);
   });
 };